Multiple layer wraparound heat shrinkable sleeves have been used for many years for sealing and protecting pipes, cables and the like. Such sleeves consist usually of a layer of crosslinked plastic sheet which is dimensionally heat unstable in one direction, on top of which is coated or laminated one or more layers of functional material. The direction of shrinkage is such that, when the sleeve is wrapped around an article, such as a pipe joint or a cable, the sleeve will, upon heating, decrease in circumference in relation to said object until it conforms tightly to the shape and profile of said object. By "functional material" is meant, as will be understood by those skilled in the art, a material which performs a function which the heat unstable sheet cannot adequately perform on its own. Most commonly a single functional material layer is used, and consists of a heat-activable polymer composition, such as a hot melt adhesive or a mastic sealant. The term "heat activable" refers to the fact that, upon heating to a moderate temperature a beneficial change in properties occurs, which will promote the function of said layer. In the case of a so-called "hot melt adhesive", the functional layer converts from what is essentially a solid, non-tacky state to what is essentially a fluid, tacky state, thus allowing it to fill in surface irregularities and form an adequate bond to the surface. In the case of a so-called "mastic sealant", the functional layer is converted from a soft gummy material which does not readily flow under the influence of gravity, to a flowable liquid which will fill in surface irregularities. Upon cooling, both types of functional material layer revert to the state they were in prior to heating. By "moderate temperatures" is meant a temperature which is not so high as to deteriorate the heat shrinkable layer, the functional material layer itself, or the substrate upon which the sleeve is being applied. Of course other types of functional material layers may be used, such as materials which crosslink at the shrinkage temperature, materials which are electrically conductive, and the like.
The heat shrinkable layer is most usually a sheet of olefinic polymer, such as polyethylene or a copolymer thereof, which has been crosslinked to render it non-melting and subsequently stretched and cooled in the stretched condition. If the temperature of the sheet is subsequently raised to or above the crystalline melting temperature of the main component material, the sheet will recover to the dimensions which it had prior to stretching.
While the design wherein the multiple layers are laminated on top of one another has provided acceptable products for many applications, it also has certain disadvantages. This is illustrated in FIGS. 1 and 2 wherein is shown a known form of wraparound sleeve 11 comprising a longitudinally heat-shrinkable backing 12, a layer of heat activable sealant or mastic 13 coated thereon, a bare area 14 and a stripe of hold down adhesive 16, as known from, for example Tailor et al U.S. Pat. No. 4,472,468. In use the sleeve 11 is wrapped around a pipe 17 and heat is applied firstly to weld an overlap end 12a of the backing at the bare area 14 to an underlap end 12b and secondly to make the layer 13 flowable and shrink down the backing 12 so that hoop stress compresses the flowable layer 13 tightly onto the surface of the pipe 17. One particular problem is that where the sleeve 11 overlaps itself there is a void 18 which must fill up during installation if a proper seal is to be achieved. This void is equal in thickness to the total thickness of the layers 12 and 13 of the laminated sleeve, as shown in FIG. 1. If absolutely no extra sealant 13 flows in to this void, it cannot be completely filled. A proper seal requires that additional sealant 13 move into the area where the void 18 had been. The higher the viscosity of the sealant 13, the more difficult this is to achieve. The presence of the heat shrinkable component 12b within this overlap region also makes it more difficult. For one thing, the heat shrinkable component 12b will not melt or flow. For another thing, the heat shrinkable layer 12 is free to recover fully in this area, with a corresponding increase in thickness which exacerbates the problem.
Commercially available systems most typically address the underlap void problem by using a sealant layer 13 which is at least as thick, and preferably at least 1.5 times as thick as the stretched heat shrinkable layer 12. Further, the sealants are most preferably formulated to give a very low viscosity at the temperatures obtained during installation. The former approach results in sleeves which are often much thicker than would otherwise be required, which means that they are more expensive, and require more heating and more time to shrink down than would otherwise be required. The latter approach limits the formulation of the sealants, confining them to relatively low molecular weights with correspondingly limited physical properties.
Another limitation of the known sleeves is that the layers must be capable of being laminated such that they will not delaminate when the sleeve is rolled or unrolled. This obviously requires that the two layers adhere to one another, but additionally this adhesion must be developed at processing temperatures which will not cause the sheet to shrink. These limitations make it difficult to apply layers of functional material which must be processed at very high temperatures, and generally limit the choices to those which form a good bond to the heat shrinkable backing.